1. Field of the Invention
The present invention relates to a method and apparatus for suppressing disturbances in the measurement of signals with a particle probe, in which a known reference voltage on a specimen is measured at at least one time.
2. Description of the Prior Art
When measuring signals with an electron probe, disturbances can occur due to contamination of the test subject and due to fluctuations of the beam current in the electron probe.
These disturbances could be nearly eliminated heretofore by a phase modulation method as discussed by H. P. Feuerbaum in the article "VLSI Testing Using The Electron Probe", Scanning Electron Microscopy, 1979, I, pp. 285-296, when such disturbances amount to only a few percent of the measured signal. The efficacy of this known method of phase modulation, however, no longer suffices. Such a known method of phase modulation, in particular, only eliminates the drift phenomena in a measured signal occurring as a result of disturbances. In general, such a measured signal not only drifts, but also becomes more inaccurate.